1. Field of the Invention
This invention relates to the manufacturing of circuit boards and electronic components, particularly to a tape feeder device for reliably conveying parts to a pickup location for attachment to a substrate using a pick and place machine. Carrier tapes used in tape feeders typically comprise a plastic strip having depressions at regular intervals containing the part to be mounted on the substrate and a second plastic cover strip covering the depressions to retain the parts in the depressions. This invention relates to a tape feeder device which can feed such carrier tapes in a variety of formats, including tapes of varying pitch, i.e., varying distances from one component to the next on the tape. Most particularly, this invention relates to a variable pitch tape feeder device which is reliable and easy to operate.
2. Description of the Related Art
The preferred method for the automated construction of circuit boards requires the use of high speed pick and place machines, which pick components from a pickup location and place them at required locations on the circuit board for attachment. Pick and place machines rely on feeding mechanisms to reliably feed the required parts to the expected pickup location. It has become common in the industry to package small electronic parts such as integrated circuit chips in carrier tape, which is characterized by a flexible strip of plastic with depressions formed at regular intervals along its length. A part is disposed in each depression and secured by a cover strip that is adhered along its edges to the carrier tape. Parts which are packaged in a carrier tape require the cover strip be peeled away from the carrier tape and that the carrier tape be advanced to bring the next part to the pickup location. Normally, the carrier tape is peeled back from the carrier tape at a point prior to the pick point as the tape is advanced.
Electronic parts are packaged in carrier tapes in a variety of formats, depending on the size of the part being delivered. In particular, carrier tapes are available in varying widths and pitches. The width is the distance from edge to edge perpendicular to the length of the tape. Widths common in the industry are 8 mm, 12 mm, 16 mm, 24 mm and up. The pitch is the distance from one depression to the next along the length of the tape. The present invention is directed to a tape feeder capable of delivering parts on 8 and 12 mm width tapes, though it can be easily modified to accommodate other sizes. The most popular pitches used by electronic parts manufacturers with 8 and 12 mm wide tapes are 2 mm, 4 mm, and 8 mm. On occasion, it is also necessary to use 12 mm pitch tape. Tapes are wound on reels and shipped to the manufacturing facility. It is obvious that the part manufacturer and user will desire to used the smallest pitch tape permissible for the size of the electronic component in order to reduce the length of the tape required, thereby reducing the size and/or number of reels for the required task.
For applications requiring high speed operation using low mass components, particularly those components with pitches of 8 mm or less, it is also important to provide means to keep each part from escaping its respective recess after the strip is removed but before it reaches the pick point. To this end, covers have been employed to cover the carrier tape past the point where the cover strip is peeled away up to the pick point.
An example is the fixed-pitch feeder shown in U.S. Pat. No. 4,327,482. Here, carrier tape T is supplied from supply reel 6. At the second roller 7, the cover strip 2 is removed and taken up by cover strip take-up reel 8. Immediately following the roller, cover 12 extends past the pick point, which is shown directly below suction head 14 at the location of a notch in the cover. This cover is not suitable for a variable pitch feeder for two reasons. First, for low mass parts, there is a risk that vibrations inherent in the advancing operation will knock the part from its recess when it reaches the pick point prior to being picked by the pick head. Second, in the case of a variable pitch feeder, the notch at the pick point must be large enough so that the largest-pitch part can pass through it, which makes it too large for smaller-pitch parts which remain exposed before they reach the pick point.
While tapes of varying widths can be easily accommodated by many feeders, providing a variable pitch feeder has proven troublesome in the industry. To accommodate carrier tapes of varying pitches, the amount of advance must be adjustable, and the size exposed area at the pickup location must be adjustable to prevent more than one part from being exposed at a time. To prevent a part from exiting its depression until it reaches the pickup location, shutter mechanisms are employed in a variety of ways.
Shutter mechanisms are used prevent a part from exiting its depression as it is being advanced to the pickup location by maintaining a cover over the depression until it reaches the pickup location. Once the part reaches the pickup location, the shutter withdraws, providing access to the exposed part so that the pick and place machine can remove it. In the case of variable pitch tape feeders, a special problem exists in that the travel distance of the shutter must vary in accordance with the pitch of the tape so that when one part reaches the pickup location, it is exposed, but the next part is not.
Several attempts have been made to provide a variable pitch tape feeder that can both vary the advance of the carrier tape and the travel distance of the shutter mechanism.
U.S. Pat. No. 5,725,140 to Weber et al. describes a variable pitch tape feeder having many of the features of the present invention. Most notably, the feeder includes adjustment means for varying the advance of the carrier tape to accommodate tapes of varying pitch as well as an adjustment for the travel distance of the shutter mechanism. Looking at FIG. 8D of this patent, to adjust the advance of the carrier tape, the user is required to rotate stop-block 130 to one of two positions and/or relocate ratchet arm 56 from pivot point 64 to pivot points S1 or S2. Additionally, to adjust the shutter-travel length, the user is required to rotate selector 139 to vary the location of pivot 97. The complicated mechanics of this device is necessary to transmit motion to all the functions of the feeder from a single actuator lever 46. While this device serves the purpose of reliably feeding carrier tapes of varying pitches, it is complicated to operate and timeconsuming to reconfigure.
U.S. Pat. No. 5,531,859 to Lee et al. and U.S. Pat. No. 5,294,035 to Asai et al. do not employ shutters, but enable selection of pitch advancement of a carrier tape by adjusting a pivot point in a manner similar to U.S. Pat. No. 5,725,140 above. For example, in U.S. Pat. No. 5,294,035,lever 74 is provided with a several pivot points 78, 88, and 89, each one of which provides for a different travel distance of plate 70 which ultimately drives feed pawl 60. Again, no shutter mechanism is provided and large rectangular opening 36 does not prevent exposed parts from escaping from their depressions due to ambient vibrations.
3. Summary of the Invention
To overcome the disadvantages of the prior art, the present invention provides a variable pitch feeder which uses an electronic control system with separate actuating mechanisms for advancing the carrier tape and the shutter mechanism. The operator is required to simply slide or rotate a selector switch to the setting which corresponds with the pitch of the carrier tape loaded in the device. In this manner, the switch serves the dual purpose of limiting rearward travel of the shutter mechanism as well as provide indication to the electronic control system of the required travel distance of the carrier tape for each advance operation. By limiting the rearward travel of the shutter mechanism, parts subsequent to the one positioned at the pick point will remain covered at all times and all the parts will be covered during the advancing operation.
In a preferred embodiment, the feeder can feed carrier tape having pitches of 2 mm, 4 mm, 8 mm, and 12 mm. The selector switch is located in an easily-accessed location behind the pickup location over the carrier tape and is actuated by sliding the switch forward or back. The switch comprises three major parts: The selector switch, a cover plate, and a shutter. The selector switch includes spring arms which engage notches in the stationary cover plate to provide 3 positive positions, each of which correlates with the pitch of the carrier tape being used. Alternatively, the selector switch includes notches which are engaged by spring arms of a stationary part. The selector switch also includes a stop surface which limits the rearward travel of the shutter and a pitch selector flag which provides, through the use of optical sensors, an indication to the electronic control unit of the desired pitch so that it will advance the carrier tape the proper distance during each advance operation. To operate at a fourth pitch, e.g., 12 mm, the entire selector assembly and shutter is removed.
In another embodiment, the selector switch is in the form of a knob which is rotated to one of four positions, each correlating to a pitch. The knob is located behind the pickup location and drives a cam to very the rearward travel limit of the shutter. In this embodiment, the knob position is advantageously sensed using magnetic Hall effect sensors to provide indication to the control unit as in the previous embodiment.